1. Field of the Invention
The present invention relates to methods of inspection usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to determine features of the substrate, such as its alignment, a beam is reflected off the surface of the substrate, for example at an alignment target, and an image is created on a camera of the reflected beam. By comparing the properties of the beam before and after it has been reflected off the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties.
Such a system of illuminating a target and collecting data from the reflected radiation is often used to illuminate a plurality of superimposed patterns. The second pattern has a predetermined bias compared to the first pattern. By analyzing the characteristics of the reflected radiation, it is possible to determine the overlay error, OV, between the gratings. A cross-section of a substrate with superimposed patterns is shown in accompanying FIG. 7. As can be seen, the exposed patterns 80, 81 are separated by a vertical height, Z, and are thus at different focal depths. It can therefore be difficult to focus on both patterns simultaneously. Consequently one of the gratings is often out of focus and a poor image results. One method of improving the focus is to increase the depth of the focus, but this decreases the amount of light detected and thus the image quality. There is therefore a trade-off between signal strength and focus depth. Furthermore, no depth information about the object is collected. An alternative method of detecting accurate images of both patterns would be to perform two measurements: one focused on each of the patterns, but this causes a significant loss in productivity.